Tube Heaters or Unit Heaters? Let the Debate Begin
This is one of the most common questions we get during the fall and winter seasons. Owners want to know what type of heating equipment they should use for their commercial warehouses and buildings, especially in their heating only applications. And for good reason; it is a tricky question.
Before we get started, keep in mind that tube heaters and unit heaters aren’t the ONLY solutions. The application might be better served with makeup air units, air rotation units, packaged rooftop units, etc. But the majority of the owners are deciding between these two solutions, so for now let’s focus here. So, should you install tube heaters or unit heaters in your space? Well…? It depends.
The short answer is, if you have a lot of air infiltration from dock doors, or have a partially outdoor space (like we do at the cfm Distributors dock), tube heaters are typically a much better solution.
Why?
It is all in how the tube heater is engineered to deliver heat to the space. Infrared tube heaters consist of two main sections, the burner box and, typically, 4″ round metal tubes that are anywhere from roughly 20′-80′ long with polished reflectors. The burner box delivers hot gas to the tubes, making the tubes extremely hot, and the heat radiates from the tubes to the space. That radiation heat warms everything in the space, except the air. It heats the floor, it heats the walls, it heats the people and everything else in the space. So even if you have a rush of cold air coming in, it still feels warm with the radiation hitting you.
We experience this same exact radiation every day from the sun. Imagine it is a brisk 45 degrees outside, which would normally feel chilly, but if the sun is out and shinning on you, you might actually be warm. Or, when you are driving in your car and it is cold outside. You probably have a jacket on and the heater turned up; but, if the sun is out, you would start to get hot after a while. These are both examples of how infrared tube heaters work.
Tube Heater Layout Design
This is a lot easier than you might think. There are just a few things to keep in mind when designing a tube heater job. Let’s say the application you are working on is a 10,000 sq/ft warehouse, with 20 dock doors. This process of designing a tube heater job can be broken down into two steps:
- Load Calculation The first thing to do is a load calculation; which, if you need help with this, please call your territory manager for assistance. We would love to help! Remember too, it is important to know when you are running your load calculation if you have decided on tube heaters or unit heaters. Why? Because with tube heaters you can maintain a lower space temperature, and the “feels like” temperature will be the same as forced air. For example, with forced air equipment you might need the space to be at 70 degrees to be comfortable. But, if you are using tube heaters, a very conservative space design temp might be 65 (or a couple of degrees less). Since your space temperature design is lower, you now have a smaller delta T with the outside; therefore, your heat load is lower.
- 2. Tube Heater Placement
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So after running your heat load you come up with 480,000 btuh required. Now we need to find the efficiency of the tube heater you are using and divide the capacity (btuhs) required by that number. For example- if the tube heater is 80% AFUE, you take 480,000 divided by 0.80 and you get 600,000 btuhs of input required. Now it is time to design the layout. How many tube heaters will you need? And where do you install them? In most cases, as long as you get the btuh’s to the space, and the heaters are evenly spaced, you will not have any issues. But here are a few things to consider when designing the layout:
- Most of the heat comes out near the burner box. The best way to visualize the heat distribution map of a tube heater with a straight tube, is to picture a Christmas tree laying on its side, and assuming the base of the tree is the burner box. You get a lot of heat at one end, and you slowly lose heat by the end of the tubes. Use this to your advantage by placing burner box ends near dock doors, outside walls, or the largest infiltration areas. If you need a more even distribution throughout the length of the tube, most manufactures have “U-Bends” that you can install. Adding a U bend will make the tube section into the shape of a U, and can even out your heat distribution.
- Watch out for clearances Check the installation manual for these distances before quoting your project. In certain capacities and tube lengths you might need 7-8′ of clearance to combustibles. Sometimes tilting the tube heater can reduce this clearance by a few feet, but double check the manual first!
- Mounting height is critical Since tube heat is radiative heat, it follows the inverse square law. When you double the height, you get 1/4 of the heat. So mounting height becomes a delicate balance between maintaining minimum clearances and being cognizant of losing effective heat when mounted too high.
But enough about tube heaters. When should you use forced air unit heaters?
When to Use Forced Air Unit Heaters
- Minimal air infiltration applications If your space will not have a lot of air infiltration, and you are looking for a more comfortable, even temperature distribution for creature comfort, unit heaters may be the best way to go. Since hot air rises, if you have really tall ceilings and you decide to go with unit heaters, consider adding some ceiling fans to keep the hot air near the floor.
- Larger and more capacity options You can also find unit heaters in much bigger sizes. Typically, the biggest tube heater you can find is 200,000 btuhs input, while standard unit heaters are available up to 400,000 btuh input. So on a 400,000 btuh application, there is a chance you could use one forced air unit heater, as opposed to two tube heaters. This will cut down on installation cost- less electrical, gas piping, roof penetrations etc.
- Lower upfront cost Upfront cost is another reason you might choose unit heaters over tube heaters. Unit heaters on average can give you up to DOUBLE the heat per dollar you spend on the equipment. So if the owner is on a very tight (upfront) budget, unit heaters will typically cost less to install. However, keep in mind that if the space is better suited for tube heaters, the unit heater option will more than likely cost more in utilities, and the owner might have to spend time warding off comfort complaints.
When it comes time to choose between tube heaters and unit heaters, the answer may not be as easy as you think. But that is why we are here, to help you make these challenging application decisions. Are you working on a project that we can help with today? Give us at cfm a call @ 1-800-322-9675, post a comment below, or message us on Facebook or Twitter @cfmdistributors. We’re always happy to assist our customers in project planning.
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